The rapid development of covalent adaptable networks or vitrimers shows promise for addressing the long-standing recycling issues associated with conventional, permanently cross-linked thermosets. At ...the same time, it is important to demonstrate that properties of reprocessable polymer networks can be optimized to meet the ongoing demand for high-performance materials. We have fabricated reprocessable polyhydroxyurethane (PHU) network composites reinforced with reactive polyhedral oligomeric silsesquioxanes (POSS). With functionalized POSS serving as a fraction of the cross-linkers, the PHU–POSS network nanocomposites exhibit significantly enhanced storage modulus at the rubbery plateau region relative to the neat PHU network. With up to 10 wt% POSS loading, these network composites can undergo melt-state reprocessing at 140 °C with 100% property recovery associated with cross-link density. We also show that hydroxyurethane dynamic chemistry leads to excellent creep resistance at elevated temperature up to 90 °C and is unaffected by reactive incorporation of POSS. This study demonstrates the effectiveness of POSS as nanofillers for designing high-performance, organic−inorganic dynamic PHU networks with excellent reprocessability.
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•Dynamic PHU–POSS network composites are made with up to 10 wt% reactive POSS.•PHU–POSS network composites can be melt-reprocessed with 100% recovery of cross-link density.•Improved thermal stability and rubbery plateau modulus relative to the neat PHU network.•Excellent elevated-temperature creep resistance comparable to static networks.
Nanoconfined polystyrene (PS) films exhibit substantial reductions in glass transition temperature (T g) from bulk T g. By incorporating 2–6 mol % 2-ethylhexyl acrylate (EHA) into styrene (S)-based ...random copolymers and characterization via ellipsometry, we show that the T g-confinement effect for films supported on silicon wafers is eliminated within experimental uncertainty down to a 15 nm thickness. Previous studies have neutralized this confinement effect by the copolymerization of minority levels of styrene with majority levels of a comonomer that can undergo hydrogen bonding with hydroxyl groups on a substrate surface, thus counteracting the free-surface-based T g reduction with a T g increase near the substrate interface. In contrast, the T g-confinement effect is eliminated in our 2–6 mol % EHA copolymers independent of the presence of substrate surface hydroxyl groups. Thus, polymer–substrate interfacial hydrogen bonds play no significant role in neutralizing the T g-confinement effect in the S-based copolymers with 2–6 mol % EHA. Instead, the neutralization must come from suppressing free-surface effects via very low levels of an EHA monomer in the copolymer. Importantly, this simple approach to eliminate the T g-confinement effects with as little as 2 mol % EHA is accompanied by only a minor change in bulk T g and no change in thermal expansivity within the experimental error relative to PS. Furthermore, this approach cannot be generalized to other acrylate comonomers, such as n-butyl acrylate. It neither requires complex syntheses to achieve dense brush, bottlebrush, or cyclic or ring polymer topologies nor the addition of a plasticizer or a surfactant to the polymer, earlier approaches that suppressed the T g-confinement effect in PS. For a 99:1 mol % S/EHA copolymer, the confinement effect is nearly identical to that of PS, indicating that a critical yet low EHA level is needed to eliminate the effect.
The rapid development of covalent adaptable networks or vitrimers shows promise for addressing the long-standing recycling issues associated with conventional, permanently cross-linked thermosets. At ...the same time, it is important to demonstrate that properties of reprocessable polymer networks can be optimized to meet the ongoing demand for high-performance materials. We have fabricated reprocessable polyhydroxyurethane (PHU) network composites reinforced with reactive polyhedral oligomeric silsesquioxanes (POSS). With functionalized POSS serving as a fraction of the cross-linkers, the PHU–POSS network nanocomposites exhibit significantly enhanced storage modulus at the rubbery plateau region relative to the neat PHU network. With up to 10 wt% POSS loading, these network composites can undergo melt-state reprocessing at 140 °C with 100% property recovery associated with cross-link density. We also show that hydroxyurethane dynamic chemistry leads to excellent creep resistance at elevated temperature up to 90 °C and is unaffected by reactive incorporation of POSS. In conclusion, this study demonstrates the effectiveness of POSS as nanofillers for designing high-performance, organic-inorganic dynamic PHU networks with excellent reprocessability.
Conventional polymer networks are composed of strong, fixed covalent cross-links. The covalent cross-links render polymer networks with outstanding mechanical properties, heat stability, and chemical ...resistance; however, they also prevent polymer networks from being decross-linked or/and recycled into similar-value products at the end of their life, leading to environmental and economic concerns. Polyurethanes (PUs) are one of the most widely used commodity polymers. However, the effective recycling of PUs, especially the cross-linked ones, has never been well established. The first part of this dissertation describes two approaches to improve the recyclability of PU and PU-like network materials. In the first approach, the intrinsic dynamic nature of urethane linkages is utilized to achieve excellent reprocessability of conventional PU networks. The property recovery of PU networks after reprocessing is enhanced by judicious molecular design, off-stoichiometry reactions, and proper choice of catalyst. In the second approach, polyhydroxyurethanes (PHUs), which are a class of non-isocyanate PU-like materials, are synthesized from bio-based precursors. Hydroxyurethane dynamic chemistry involves dual dissociative and associative mechanisms. Exploiting this dynamic chemistry, we develop bio-based PHU networks that exhibit complete recovery of cross-link density and tensile properties after multiple reprocessing cycles. We also demonstrate that the mechanical and thermal properties of reprocessable PHU networks can be enhanced by using polyhedral oligomeric silsesquioxanes (POSS) with cyclic carbonate end groups as reactive nanofillers to fabricate dynamic PHU network composites. The intrinsic reprocessability of PHU networks is not impacted by POSS incorporation. With up to 10 wt% POSS loading, PHU–POSS network composites can undergo multiple reprocessing cycles with 100% recovery of cross-link density. The second part of this dissertation focuses on understanding the residual stress relaxation behavior and stiffness of supported acrylic random copolymer films. The residual stress relaxation process in spin-coated poly(styrene/n-butyl acrylate) (P(S/nBA)) random copolymer films is characterized by ellipsometry and fluorescence. Both techniques show that stress relaxation occurs over a period of hours in the rubbery state and that the presence of a very small amount of nBA units vastly changes the relaxation behavior from neat polystyrene films. Fluorescence characterizations show that the interfacial stiffness of supported P(S/nBA) films is enhanced by attractive hydrogen bonding interactions between polymer films and substrates. The length scale over which substrate perturbations modify the average stiffness of a P(S/nBA) film near a hydrophilic substrate interface increases with increasing nBA molar content. In contrast, studies associated with supported poly(styrene/2-ethylhexyl acrylate) (P(S/EHA)) random copolymer films reveal that the interfacial stiffness and stiffness gradient length scales in supported P(S/EHA) films are independent of interfacial hydrogen bonding interactions. For P(S/EHA)s and P(S/nBA)s with similar bulk glass transition temperatures, the stiffness gradient length scales associated with P(S/EHA) films are substantially longer than those of P(S/nBA) films.
Tungsten (W)-based composites with 1 wt% SiC were consolidated via field assisted sintering (FAST). FAST was conducted at temperatures ranging from 1000 °C to 1700 °C with holding times of 1–10 min ...under a pressure of 50 MPa. The densification and grain growth mechanisms were investigated using a creep deformation model and the grain growth power law. Various densification and grain growth mechanisms corresponded to different effective stress exponents (n) and grain growth exponents (p). n is the stress exponent associated with the densification mechanism, and p is the grain growth exponent related to the grain growth mechanism. The densification process mainly occurred at 1000 °C–1400 °C, whereas grain growth mainly occurred at 1500 °C–1700 °C. The W-Si-C composites were densified primarily through particle rearrangement (when n is ∼1), intergranular diffusion (when n is ∼2), and the dislocation-climb mechanism (when n is 3.62). The activation energies were estimated to be 153.80 and 541.17 kJ/mol at the effective stress exponents (n) of 1 and 2, respectively. Furthermore, intergranular diffusion is speculated to cause grain growth. The sintering temperature considerably affected the microstructure of W-Si-C composites. The silicide formed at sintering temperatures of <1100 °C was WSi2, and it transformed into W5Si3 with increasing sintering temperature.
•Densification mechanism of W-Si-C (W + 1 wt% SiC) composites was investigated by creep deformation model.•The densification mechanism is particle rearrangement, intergranular diffusion and dislocation creep mechanisms.•The addition of SiC can promote the densification of tungsten due to the low apparent activation energy.•Grain growth mechanism of W-Si-C composites was investigated, grain growth was related to grain boundary diffusion.
Dynamic urethane chemistry, which involves both associative and dissociative mechanisms, can be used to achieve reprocessability in cross-linked polyurethane (PU) networks. We discovered that ...property recovery and thermal stability of dynamic covalent PU networks can be enhanced by incorporating excess free hydroxyl groups, thus performing polymerization reactions at slight stoichiometric imbalance of hydroxyl and isocyanate groups, and by increasing the cross-linker functionality from three to four. While free hydroxyl groups suppress the reversion of urethane links and minimize side reactions associated with liberated isocyanate groups under reprocessing conditions, tetrafunctional cross-linkers help to maintain network integrity in the presence of small levels of side reactions. Using these strategies, we developed a PU network that, within experimental error, exhibits 100% recovery of cross-link density and tensile properties after multiple reprocessing steps, which has not been reported before. We also demonstrated the potential of recovering monomer from PU networks by alcoholysis under relatively mild conditions. Our study not only offers a solution to the long-standing issue of PU network recycling but also provides simple strategies to improve property recovery of reprocessable networks.
The heterogeneous nucleation of colloidal crystals with attractive interactions has been investigated via in situ observations. We have found two types of nucleation processes: a cluster that ...overcomes the critical size for nucleation with a monolayer, and a method that occurs with two layers. The Gibbs free energy changes (ΔG) for these two types of nucleation processes are evaluated by taking into account the effect of various interfacial energies. In contrast to homogeneous nucleation, the change in interfacial free energy, Δσ, is generated for colloidal nucleation on a foreign substrate such as a cover glass in the present study. The Δσ and step free energy of the first layer, γ1, are obtained experimentally based on the equation deduced from classical nucleation theory (CNT). It is concluded that the ΔG of q-2D nuclei is smaller than of monolayer nuclei, provided that the same number of particles are used, which explains the experimental result that the critical size in q-2D nuclei is smaller than that in monolayer nuclei.
Understanding nucleation dynamics is important both fundamentally and technologically in materials science and other scientific fields. Two-dimensional (2D) nucleation is the predominant growth ...mechanism in colloidal crystallization, in which the particle interaction is attractive, and has recently been regarded as a promising method to fabricate varieties of complex nanostructures possessing innovative functionality. Here, polymers are added to a colloidal suspension to generate a depletion attractive force, and the detailed 2D nucleation process on the terrace of the colloidal crystals is investigated. In the system, we first measured the nucleation rate at various area fractions of particles on the terrace, ϕarea. In situ observations at single-particle resolution revealed that nucleation behavior follows the framework of classical nucleation theory (CNT), such as single-step nucleation pathway and existence of critical size. Characteristic nucleation behavior is observed in that the nucleation and growth stage are clearly differentiated. When many nuclei form in a small area of the terrace, a high density of kink sites of once formed islands makes growth more likely to occur than further nucleation because nucleation has a higher energy barrier than growth. The steady-state homogeneous 2D nucleation rate, J, and the critical size of nuclei, r*, are measured by in situ observations based on the CNT, which enable us to obtain the step free energy, γ, which is an important parameter for characterizing the nucleation process. The γ value is found to change according to the strength of attraction, which is tuned by the concentration of the polymer as a depletant.
We investigate the usage of polyelectrolyte complex materials for water remediation purposes, specifically their ability to remove nanoplastics from water, on which there is currently little to no ...prior research. We demonstrate that oppositely charged random copolymers are effective at quantitatively removing nanoplastic contamination from aqueous solution. The mechanisms underlying this remediation ability are explored through computational simulations, with corroborating quartz crystal microbalance adsorption experiments. We find that hydrophobic nanostructures and interactions likely play an important role.
Impurity partitioning during colloidal crystallization was investigated for grains with different orientations. Particles of various sizes were doped as impurities during the growth of colloidal ...polycrystals. The effective partition coefficient, keff, which is the impurity concentration in the solid (CS) divided by that in initial solution (CL), was measured for grains oriented in the 111 and 100 directions normal to the growth direction. The 111-oriented grains were found to have a larger keff than 100-oriented grains. This was analyzed by using the Thurmond and Struthers model. Though both 111- and 100-oriented grains were face centered cubic (fcc) structures, within several layers of crystals, the volume fraction of 111-oriented grains was larger than that of 100-oriented grains, yielding a larger driving force for nucleation, ΔGTr, and thus a larger equilibrium partition coefficient, k0, for 111-oriented grains.
•The 111-oriented grains were found to have a larger keff than 100-oriented.•The volume fraction of 111-oriented grains is larger than that of 100-oriented.•Larger ϕTr for 111-oriented grains leads to larger k0 than for 100-oriented.
